In a variable air volume (VAV) temperature conditioning system, a plurality of comfort zones are maintained at a setpoint temperature by regulation of the flow of temperature conditioned supply air into each of the zones. Controls for a VAV system are generally designed to maintain the supply air at a relatively constant temperature, in the range of 50.degree. to 60.degree. F. VAV systems are thus normally used for cooling only applications, as for example in cooling the interior zones of large buildings. However, VAV systems equipped for automatic changeover from heating to cooling have been used successfully in application to single story buildings of less than 6,000 square feet, having total cooling requirements of 15 tons or less. In a relatively small building, it is less likely that there would be an interior zone requiring cooling at the same time an exterior zone required heating. VAV systems are generally not used in heating only applications, since constant volume systems are typically more efficient.
A conventional VAV system includes controls which energize and de-energize stages of temperature conditioning in response to the deviation of the supply air temperature from a setpoint. To insure more responsive control, means may also be provided to reset the supply air temperature setpoint in proportion to the mixed, return and fresh air temperature. For example, using a reset ratio of 1:5, if the mixed air temperature decreases by 5.degree. F., the supply air temperature setpoint would be increased by 1.degree. F. The reset ratio is selected to optimize the balance between compressor energy consumption and the indoor fan energy consumption. Anti-cycling timers are also included in a conventional VAV system control to prevent damage to the compressors which might otherwise result from too rapid cycling, and to allow time for the system to react to changes in staging of the temperature conditioning apparatus.
One of the disadvantages of the prior art VAV system controls is that by responding to deviations of the supply temperature from a setpoint, the controls energize stages of temperature conditioning sooner and more often than may be necessary to meet the true temperature conditioning demand. This may result in excessive cycling and excessive energy consumption by the temperature conditioning apparatus. Prior art controls which reset the supply air temperature setpoint in proportion to the mixed, return and fresh air temperature are less susceptible to this problem, but nevertheless fail to control staging in response to temperature conditioning demand. These controls provide the coldest supply air when the demand for cooling is low, instead of when it is high.
Since a heat pump provides both heating and cooling capability, it would not normally be used in a VAV system. However, where heating-only, or both heating and cooling capability are desired, a heat pump provides an energy efficient alternative to a fuel oil or natural gas furnace installation. Unfortunately, the heating efficiency of an ambient air source heat pump declines significantly as the outdoor temperature drops, making it necessary to provide auxiliary heating stages for operation at low ambient temperatures. A control for a VAV system which includes a heat pump should therefore properly provide means to match the available heating capacity of the system with the heating demand, such that staging is controlled as a function of the outdoor ambient temperature. The prior art VAV control systems do not provide means to accomplish this function.
Under certain conditions, an indoor heat exchange coil in a cooling system may develop a layer of frost and ice on its surface, greatly reducing its cooling capacity due to the restricted air flow across its surface. Under these circumstances, it is necessary to provide means to defrost the coil. Various techniques have been developed in the prior art to detect a frost covered indoor heat exchanger and to defrost it. Hot gas bypass, a process in which hot gas from the compressor is applied to the indoor heat exchanger, bypassing the outdoor condensing coil is a common method of preventing frosting of the coil by maintaining a minimum saturated suction temperature above 30.degree. F. This method for preventing frost formation and other prior art means for defrosting a heat exchange coil waste energy and, in terms of hardware, are expensive to provide.
In consideration of the problems described above, it is an object of this invention to provide a more efficient control for a temperature conditioning system having a plurality of stages for temperature conditioning air supplied to comfort zones wherein the rate of flow of that air is regulated so as to maintain a setpoint temperature in each of those zones.
It is a further object of this invention to minimize energy consumption and cycling of the temperature conditioning system by controlling the temperature conditioning apparatus thereof in response to temperature conditioning demand, such that the maximum absolute magnitude of the difference of the return air and the supply air temperature occurs in response to high temperature conditioning demand.
A still further object of this invention is to provide means and method for controlling heating apparatus in response to a heating demand, taking into consideration that the heating capacity of that apparatus is a function of the outdoor ambient air temperature.
Yet a still further object of this invention is to detect the need to defrost the indoor heat exchanger by sensing the suction line temperature; and to defrost the indoor heat exchanger by de-energizing the stages of cooling then in operation and circulating return air through the heat exchanger to melt the ice and frost thereon, and thereby cool the air supplied to the comfort zone.
Still a further object of this invention is to provide means and method for controlling the mode in which the temperature conditioning system operates, i.e., in a heating, cooling, or ventilation mode, as a function of the outdoor ambient temperature, and enthalpy.
These and other objects of the subject invention will become apparent from the description which follows and by reference to the attached drawings.